1. Field of the Invention
This invention pertains to a process for making chalcogenide waveguides and to low loss chalcogenide glass waveguides, including chalcogenide glass fibers, produced thereby.
2. Description of Related Art
Silica and fluoride optical fibers have a limited usage beyond 2 μm and 3 μm, respectively, since these fibers exhibit significant multiphonon absorption. On the other hand, chalcogenide glasses transmit to beyond 12 μm in the infrared (IR).
The IR transmitting chalcogenide glasses and optical fibers encompass the IR region of interest with numerous applications including thermal imaging, temperature monitoring, and medical applications, including high energy IR laser power delivery such as CO (5.4 μm) and CO2 (10.6 μm) lasers. In addition, these fibers are being developed for remote fiber optic chemical sensor systems for military and industrial facility clean-up and other military and industrial applications. More recently, there is an interest in developing rare-earth doped chalcogenide fibers for 1.3 μm fiber amplifiers for telecommunications as well as sources in the IR.
To date, the conventional process for making the chalcogenide glasses is to use elemental precursor, such as arsenic, and a chalcogenide, such as sulfur, batched in a sealed silica ampoule. The conventional process involves high temperature of greater than about 750° required for melting and homogenization of the elemental arsenic and sulfur. The high temperature melting/homogenization process causes the precursor arsenic and sulfur to react with the silica ampule thus introducing undesired contamination into the glass and fiber. Contamination reduces fiber strength and contributes to additional extrinsic scattering loss in the fiber.
Therefore, there is a need for a new approach to melt and homogenize the arsenic and sulfur, or their counterparts, at lower temperatures. The objective of this invention is to make arsenic monosulfide compound, or its counterpart, at lower temperatures, typically between 350° C. and 400° C. The attractive properties of arsenic monosulfide (A4 S4), or another counterpart, include low melting point and low chemical activity. Because the stable arsenic monosulfide compound, or its counterpart, has a low chemical activity, by adding appropriate amount of sulfur, or another chalcogenide, the arsenic monosulfide and sulfur precursors, or their counterparts, can be dynamically distilled at a lower temperature, such as between 420° C. and 450° C., and remelted and homogenized at a lower temperature, such as 600° C. This eliminates reaction between arsenic and sulfur, or their counterparts with silica ampoule.
The Russian patent with a filing number of 4808456/33 was filed Apr. 2, 1990, and discloses a process for making a chalcogenide glass fiber that can be used for transmission of laser energy in applications such as laser surgery, in making instruments for industrial diagnosis of electronic devices, and the like. The object of the process is reduction of optical losses which is achieved by using arsenic monosulfide (As4 S4) in place of arsenic and sulfur to make chalcogenide glass and glass waveguides, such as glass fibers. In a particular embodiment disclosed by the Russian patent noted above, about 600 g arsenic monosulfide was evaporated in a closed ampule under pressure at 550° C. of which about 540 g of of purified arsenic monosulfide was deposited at the cold end of the ampoule following which, 81 g of sulfur was added to the 540 g of the arsenic monosulfide in the ampoule and the mixture was melted and reacted at 550° C. for 10 hours in a sealed ampoule. Rate of evaporation of the mixture was 0.9×10−3 g/cm2-s and glass fibers were drawn from the glass prepared as described, which had optical loss of 40-100 dB/km in the 2-8 μm wavelength region when the evaporation/distillation rate was (0.8-1.0)×10−3 g/cm2-sec and varied greatly at distillation rates slightly above or slightly below the (0.8-1.0)×10−3 g/cm2-sec.